Conventionally, when a semiconductor substrate such as a semiconductor wafer (that can be abbreviated to “wafer”) is conveyed between a cassette that can accommodate the wafer and a semiconductor manufacturing unit or between various semiconductor manufacturing units, a conveying arm that forms a conveying unit is used.
A multi-chamber type of processing unit is explained as an example, which is also a cluster type of unit. In the case, a conveying arm is accommodated in a transfer-chamber (common transfer-chamber). The conveying arm has a substrate holding portion. The substrate holding portion is formed in such a manner that the substrate holding portion can be moved into not only the transfer-chamber but also various process-chambers or a cassette-chamber that are connected to the transfer-chamber. Thus, when the conveying arm is operated, an unprocessed wafer can be conveyed from a cassette in the cassette-chamber to any of the process-chambers through the transfer-chamber. In addition, when the conveying arm is operated, a processed wafer can be conveyed back from any of the process-chambers to the cassette through the transfer-chamber.
As the conveying arm, a scalar-type of conveying arm and a frog-leg type of conveying arm are used in general.
The scalar-type of conveying arm has two or more arms that connect a substrate holding portion and a pivotable shaft. A pivoting mechanism is connected to the pivotable shaft and respective pivotable portions of the arms, for example via one or more rotating belts and one or more pulleys. Thus, when the pivotable shaft is operated, the arms can extend or retract in a horizontal direction and the substrate holding portion can be moved to predetermined positions.
On the other hand, the frog-leg type of conveying arm has a plurality of frog-leg arms, each of which consists of a pair of arms that are connected with each other substantially in a frog-leg manner. The plurality of frog-leg arms connects a substrate holding portion and a pivotable shaft. A pivoting-power transmitting mechanism is connected to the pivotable shaft. When the pivotable shaft is operated, the plurality of frog-leg arms can extend or retract in a radial direction from a center of the pivot and the substrate holding portion can be moved to predetermined positions.
When the scalar type of conveying arm is adopted for the above processing unit, the rotating belts and the pulleys, which form the pivoting mechanism, have to be disposed in the transfer-chamber through which the wafer is moved. Thus, when the conveying arm is operated, particles may tend to be generated. The particles may easily contaminate the wafers and may deteriorate a yield of the wafers.
In addition, in the scalar type of conveying arm, vibrations may tend to be generated because the scalar type of conveying arm has the rotating belts and the pulleys. Thus, it is difficult to raise a conveying speed.
When the frog-leg type of conveying arm is adopted for the above processing unit, the above rotating belts or pulleys generally need not be disposed in the transfer-chamber. Thus, it can be prevented or inhibited that particles are generated and that vibrations are generated. Thus, it is possible to raise a conveying speed.
However, because of the structure itself of the frog-leg type of conveying arm, an end portion of the arms connected to the substrate holding portion can not retract in a radial direction of a side substantially opposite to the substrate holding portion with respect to the center of the pivot of the whole conveying arm. That is, there is a limitation to enlarge an extending/retracting distance, in which the conveying arm can extend and retract, with respect to a pivoting radius of the conveying arm. Thus, when the center of the pivot is away from a position to which a wafer should be moved, the conveying arm has to be proportionally enlarged. Therefore, it is impossible that the unit adopting the conveying arm is miniaturized.
In addition, if the substrate holding portion is arranged further away from the center of the pivot, the substrate holding portion is subjected to a larger centrifugal force when the conveying arm is pivoted. Thus, because of the centrifugal force and/or vibrations caused by the centrifugal force, the wafer placed on the substrate holding portion may be moved out from a predetermined position or may even fall down.
In addition, in both of the scalar type of conveying arm and the frog-leg type of conveying arm, a resolution in a state wherein the conveying arm relatively contracts is very different from a resolution in another state wherein the conveying arm relatively extends. That is, when the conveying arm is caused to extend or contract in the state wherein the conveying arm relatively contracts, a resolution by which the substrate holding portion is moved in a radial direction is relatively high. On the other hand, when the conveying arm is caused to extend or contract in the state wherein the conveying arm relatively extends, a resolution by which the substrate holding portion is moved in the radial direction is relatively low. Thus, it is difficult to exactly convey a wafer placed on the substrate holding portion to a predetermined position. In particular, if the conveying arm is operated at a higher speed, it becomes more difficult to precisely convey the wafer.